REVENUE (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of revenue [Line Items]
Revenue	$ 204,575	$ 234,979	$ 620,569	$ 726,147
Revenue recognized that was included in contract liability at the beginning of the period	6,100		17,400
Service revenue
Schedule of revenue [Line Items]
Revenue	196,906	224,308	594,533	692,254
Reimbursable expenses
Schedule of revenue [Line Items]
Revenue	6,815	9,866	23,970	31,786
Non-controlling interests
Schedule of revenue [Line Items]
Revenue	$ 854	$ 805	$ 2,066	$ 2,107